Copy source files into source map file

[verhovsky]

Closes #22189

[verhovsky]

To test this on M1 macOS, I had to

brew install llvm
git checkout f2568054e3ab2251647e87c8bd651373e1b003b7

make this edit

diff --git a/tools/building.py b/tools/building.py
index 900918b44..d2fc19d60 100644
--- a/tools/building.py
+++ b/tools/building.py
@@ -39,7 +39,7 @@ logger = logging.getLogger('building')
 
 #  Building
 binaryen_checked = False
-EXPECTED_BINARYEN_VERSION = 115
+EXPECTED_BINARYEN_VERSION = 117
 
 _is_ar_cache: Dict[str, bool] = {}
 # the exports the user requested

then

./bootstrap
make
./emcc --generate-config

then edit .emscripten and change LLVM_ROOT to this

LLVM_ROOT = '/opt/homebrew/opt/llvm/bin' # directory

then

make
./emcc --check

Then I did

cd ..
git clone https://github.com/tree-sitter/tree-sitter.git
cd tree-sitter
git checkout 3abb1047

made this edit there

 script/build-wasm | 1 +
 1 file changed, 1 insertion(+)

diff --git a/script/build-wasm b/script/build-wasm
index d22b5875..c1a37414 100755
--- a/script/build-wasm
+++ b/script/build-wasm
@@ -117,6 +117,7 @@ exported_functions=$(
 
 # Use emscripten to generate `tree-sitter.js` and `tree-sitter.wasm`
 # in the `target/scratch` directory
+emcc=../emscripten/emcc
 $emcc                                            \
   -s WASM=1                                      \
   -s INITIAL_MEMORY=33554432                     \

then ran

./scripts/build-wasm

and it turns out that sourcesContent is removed by some other step (binaryen?) after if it's written with --sources in wasm-sourcemap.py with the fixes above.